Why would Topamax cause stomach pain/nausea?

I've been on Topamax for a year now, and stopped taking it for about a month when I ran out. To be honest, I didn't notice any difference, but started taking it again about 2 weeks ago just in case. The day I started taking it, I came very close to being sick (although I thought that was due to seeing my fiance post-surgery) and a couple days later started feeling nauseated and having stomach pain constantly. This is how I felt when I had ulcers a year ago, except that was a couple isolated incidents and this is non-stop. I can't eat more than a few bites, and even water hurts to drink. I read that it's worth telling a doctor if you have stomach/abdominal pain after taking Topamax, and I'm wondering if 1) I should tell my Dr. and 2) why it might cause this (what the mechanism is). I called the doctor when I first started having the pain, and she said it sounded like the ulcers and that she wouldn't even see me; my gastroenterologist should see me (and he will at the end of March!).